Aspheric lenses focus or collimate light without introducing spherical aberration into the transmitted wavefront. For monochromatic sources, spherical aberration often prevents a single spherical lens from achieving diffraction-limited performance when focusing or collimating light. Thus, an aspheric lens is often the best single element solution for many applications including collimating the output of a fiber or laser diode, coupling light into a fiber, spatial filtering, or imaging light onto a detector.

All of these molded glass lenses are also available premounted in non-magnetic 303 stainless steel lens cells that are engraved with the part number for easy identification. These mounted aspheres have a metric thread that makes them easy to integrate into an optical setup or OEM application. The mounted aspheres are readily adapted to our SM1-threaded (1.035"-40) of lens tubes by using our Aspheric Lens Adapters. Mounted aspheres can be used as a drop-in replacement for multi-element microscope objectives by combining the lens with our Microscope Objective Adapter Extension Tube.

If an unmounted aspheric lens is being used to collimate the light from a point source or laser diode, the side with the greater radius of curvature (i.e., the flatter surface) should face the point source or laser diode. To collimate light using one of our mounted aspheric lenses, orient the housing so that the externally threaded end of the mount faces the source.

Molded glass aspheres are manufactured from a variety of optical glasses to yield the indicated performance. The molding process will cause the properties of the glass (e.g., Abbe number) to deviate slightly from those given by glass manufacturers. Specific material properties for each lens can be found by clicking on the Glass link in the tables below.

Choosing a Lens

Aspheric lenses are commonly chosen to couple incident light with a diameter of 1 - 5 mm into a single mode fiber. A simple example will illustrate the key specifications to consider when trying to choose the correct lens.

The specifications for the P1-630A-FC-2, 630 nm, FC/PC single mode patch cable indicate that the mode field diameter (MFD) is 4.3 μm. This specification should be matched to the diffraction-limited spot size given by the following equation:

Here, f is the focal length of the lens, λ is the wavelength of the input light, and D is the diameter of collimated beam incident on the lens. Solving for the desired focal length of the collimating lens yields

Thorlabs offers a large selection of mounted and unmounted aspheric lenses to choose from. The aspheric lens with a focal length that is closest to 16 mm has a focal length of 15.29 mm (Item# 354260-B or A260-B). This lens also has a clear aperture that is larger than the collimated beam diameter. Therefore, this aspheric lens is the best option given the initial parameters (i.e., a P1-630A-FC-2 single mode fiber and a collimated beam diameter of 3 mm). Remember, for optimum coupling the spot size of the focused beam must be less than the MFD of the single mode fiber. As a result, if an aspheric lens is not available that provides an exact match, then choose the aspheric lens with a focal length that is shorter than the calculation above yields. Alternatively, if the clear aperture of the aspheric lens is large enough, the beam can be expanded before the aspheric lens, which has the result of reducing the spot size of the focus beam.

The target values of these constants are available by clicking on the Info Icons below or by viewing the .pdf and .dxf files available for each lens. Links to the files can be found under the Drawings and Documents tab or by clicking on the part number in the price tables below.

Aspheric Lens Design Formula

Positive Radius Indicates that the Center of Curvature is to the Right of the Lens

Negative Radius Indicates that the Center of Curvature is to the Left of the Lens

Aspheric Lens Equation

Choosing a Collimation Lens for Your Laser Diode

Since the output of a laser diode is highly divergent, collimating optics are necessary. Since aspheric lenses do not introduce spherical aberration, they are commonly chosen when the collimated laser beam is to be between one and five millimeters. A simple example will illustrate the key specifications to consider when choosing the correct lens for a given application.

The specifications for the L780P010 laser diode indicate that the typical parallel and perpendicular FWHM beam divergences are 10° and 30°, respectively. Therefore, as the light diverges, an elliptical beam will result. To collect as much light as possible during the collimation process, consider the larger of these two divergence angles in any calculations (i.e., in this case use 30°). If you wish to convert your elliptical beam in to a round one, we suggest using an Anamorphic Prism Pair, which magnifies one axis of your beam.

Ø = Beam Diameter

Θ = Divergence Angle

From the information above, the focal length of the lens can be determined, using the thin lens approximation:

With this information known, it is now time to choose the appropriate collimating lens. Thorlabs offers a large selection of aspheric lenses to choose from. For this application the ideal lens is a -B AR-coated molded glass aspheric lens with focal length near 5.6 mm. The C171TMD-B (mounted) or 354171-B (unmounted) aspheric lenses have a focal length of 6.20 mm, which will result in a collimated beam diameter (major axis) of 3.3 mm. Next, check to see if the numerical aperture (NA) of the diode is smaller than the NA of the lens:

0.30 = NALens > NADiode ≈ sin(15°) = 0.26

Up to this point, we have been using the FWHM beam diameter to characterize the beam. However, a better practice is to use the 1/e2 beam diameter. For a Gaussian beam profile, the 1/e2 diameter is almost equal to 1.7X the FWHM diameter. The 1/e2 beam diameter therefore captures more of the laser diode's output light (for greater power delivery) and minimizes far-field diffraction (by clipping less of the incident light).

A good rule of thumb is to pick a lens with an NA twice of the NA of the laser diode. For example, either the A390-B or the A390TM-B could be used as these lenses each have an NA of 0.53, which is more than twice the approximate NA of our laser diode (0.26). Note that these lenses each have a focal length of 4.6 mm, resulting in an approximate major beam diameter of 2.5 mm.

AR Coating Abbreviations

Abbreviation

Description

U

Uncoated: Optics do not have an AR Coating of any kind

A

Broadband AR Coating for the 350 - 700 nm or 400 - 600 nm range

B

Broadband AR Coating for the 600 - 1050 nm or 650 - 1050 nm range

C

Broadband AR Coating for the 1050 - 1620 nm range

V

Narrowband AR Coating designed for the wavelength listed in the table below

The table below contains all molded visible and near-IR aspheric lenses offered by Thorlabs. For our selection of IR molded aspheres, click here. The item # listed is that of the unmounted, uncoated lens. An "X" in any of the five AR Coating Columns indicates the lens is available with that coating (note that the V coating availability is indicated with the design wavelength). The table to the right defines each letter and lists the specified AR coating range. Click on the linked X's to purchase the specific lens, which is available mounted and unmounted.

The mounted working distance is measured from the edge of the unthreaded portion of the housing.

The working distance is measured to the edge of the laser diode window (instead of the emission point).

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Posted Comments:

Poster:mbennahmias

Posted Date:2016-06-27 10:54:25.437

Hello,
I would like to know what is the laser induced damage threshold for this optical element / AR coating.
I am using a 0.2 W 375 nm laser diode and the diverging beam size at this location is ~ 0.1 to 0.2 mm

Poster:andrew.logan

Posted Date:2014-12-16 13:52:10.44

In the drawings for the 354560-A you give the tolerance on the lens diameter at +/- 0.15 mm, whereas Lightpath's 2014 pdf catalogue shows their typical tolerance as +/- 0.015 mm. Is this a typo, or does the 354560 have a tolerance that is an order of magnitude worse than what is typical of Lightpath?
Thanks

Poster:myanakas

Posted Date:2015-01-14 02:19:40.0

Response from Mike at Thorlabs: Thank you for your feedback. This is a typo, the correct tolerance is +/- 0.015. We are currently working to have the website updated to correct this.

Poster:moritz.kick

Posted Date:2014-01-07 15:25:08.0

Hi,
we want to couple a laser beam into and out of a fiber. The wavelength is 580 nm, we use the 460-HP fiber (MFD 3.5µm) and the Collimated Beam Diameter prior to Lens is about 1.3 mm.
I calculated f as 6.16 so the best aspheric lens for coupling the light into the fiber would be the item 352170-A. Is this correct so far?
But what kind of lense do I need to collimate the beam past the fiber. Can I use the same lense for both sides?
Thanks for your support

Poster:jlow

Posted Date:2014-01-08 05:10:48.0

Response from Jeremy at Thorlabs: At 580nm, the estimated MFD of the 460HP fiber is around 3.9µm nominally. Therefore, the ideal focal length lens would have around 6.8mm focal length. Using that number, the closest focal length asphere that would give you the best theoretical coupling efficiency would be the A375-A (7.5mm focal length). However, the difference with using the 6.24mm focal length asphere is relatively small (<0.5% absolute). For the collimation on the other end, it depends on the beam diameter and beam divergence you are looking to have on the output. Longer focal length would give you larger beam diameter and smaller beam divergence. You can use the same focal length lens on the output as well.

Poster:tcohen

Posted Date:2012-09-04 10:05:00.0

Response from Tim at Thorlabs: Thank you for contacting us. For the most efficiency in our support, a member of our China support team will contact you directly.

Response from Buki at Thorlabs.com
Thank you for participating in our Feedback Forum. We can provide custom coatings on our lenses. We have contacted you to get more information from you in order to provide a quote.